Seasonal persistence and interannual declines in mahimahi ( Coryphaena hippurus ) availability across multiple fisheries in the central North Pacific

Abstract

Mahimahi (Coryphaena hippurus) are fast-growing, short-lived pelagic predators that have long supported fisheries and subsistence harvest in tropical regions, particularly Hawaiʻi, and remain an important component of present-day fisheries. Despite their importance, research on catch patterns and oceanographic associations in the central North Pacific has been limited in recent decades. Here, we integrate multiple decades of data from two central North Pacific pelagic fisheries—a nearly basin-scale longline fishery and Hawaiʻi’s near-island troll fishery—to examine variability in mahimahi catch rates and evaluate whether hypotheses proposed near the turn of the century are supported by the greater spatial and temporal footprint of modern fisheries data. We developed generalized additive mixed models to evaluate the influence of oceanography and fishery dynamics on catch rates, examined nominal and model-derived seasonal patterns, and evaluated whether previously postulated thermal constraints remain relevant to mahimahi availability. Temperature and water clarity played a role in longline catch rates, although fisher effects remained important, whereas near-island troll catch rates were driven primarily by fisher behavior. Across both fisheries, catch rates exhibited persistent seasonality, with peaks in spring and fall shifting toward summer north of Hawaiʻi and toward winter south of Hawaiʻi. These findings support a previously hypothesized conceptual model of meridional migration in which mahimahi distribution expands northward in summer and contracts southward in winter. Relative availability indices revealed declining interannual trends in mahimahi availability for both fisheries. The position of the 23°C isotherm, hypothesized in the 1990s to influence mahimahi distribution, was significantly related to interannual variability in mahimahi availability only in the northern domain of the longline fishery in recent decades. This may be due to the poleward shift of the isotherm connected to larger subtropical gyre dynamics. Together, these findings highlight how migration and basin-scale variability shape mahimahi dynamics across the central North Pacific, providing an evaluation of existing hypotheses, and emphasizing the value of multi-scale perspectives for understanding highly migratory pelagic species.